There is a need to separate strain and temperature for fiber sensing. Foil strain gauges may be used to measure the strain on a mechanical structure. FIG. 1 is an illustration of a foil gauge, which because of its flower petals resemblance, is often called a foil gauge rosette. Foil strain sensors show similar cross sensitivities primarily due to the dominance of the thermal expansion coefficient of the material under test in most testing circumstances. On a structure where the orientation of the principal strain axis is unknown, a foil strain gauge rosette may be used to measure strain in two or three axes and thus determine the principal strains. A rosette of three gauges may be created by placing gauges at 0, 45, and 90 degrees as shown in FIG. 1.
By measuring the strain components along three different axes, common mode effects, such as those due to temperature changes in the material under test, can be removed from the strain measurement. A typical foil gauge rosette employs three strain gauges, each requiring three electrical connections for best use. This configuration can be tedious and bulky if multiple measurement points are required.
The inventors in this application realized that high resolution (along the length of the fiber) and high accuracy (in strain) strain measurements possible using modern fiber optic sensing systems means that curved lengths of fiber can be used to measure complex strain fields. Advantages of using curved fiber include smaller and more flexible sensors along with easy production and installation of same.